Production of copper and copper oxide powder for powder metallurgy

ABSTRACT

Copper base (including pure copper and copper alloy) or copper oxide powder is produced by atomizing a sulfurized copper base melt in the presence of oxygen to produce particles which are hollow as a result of sulfur dioxide formation therewithin, oxidizing the hollow particles in the presence of oxygen in order to cause weakening at the grain boundaries as a result of more sulfur dioxide formation therein, and crushing in order to make available the copper base particles. If desired, the oxidizing step, when carried for a longer time, results in oxidation of copper as well as sulfur and the crushing step results in copper oxide particles.

United States Patent 1 1 1 1 ,866 Nayar Oct. 16, 1973 1 PRODUCTION OFCOPPER AND COPPER 3,293,006 12/1966 Bartz 7s/o.5 R

OXIDE POWDER FOR POWDER METALLURGY Primary ExaminerW. W. Stallard [75]Inventor: Harbhajan S. Nayar, Waltham, Attorney-Morse, Altman & OatesMass.

[73] Assignee: Contemporary Research llnc.,

Natick, Mass. [57] ABSTRACT [22] Filed: 1971 Copper base (including purecopper and copper alloy) [21] Appl. No.: 79,355 or copper oxide powderis produced by atomizing a sulfurized copper base melt in the presenceof oxygen Related Apphcanon Data to produce particles which are hollowas a result of Division of 1 1 Sept- 9, 1963, sulfur dioxide formationtherewithin,- oxidizing the hollow particles in the presence of oxygenin order to cause weakening at the grain boundaries as a result of [52]US. Cl 75/05 B more Sulfur dioxide formation therein, and crushing in[51] Int. Cl 1322f 9/00 Order to make available the copper baseparticles If [58] Field of Search 75/0.5 B desired, the idi i step, whencarried for a longer time, results in oxidation of copper as well assulfur (56] References C'ted and the crushing step results in copperoxide particles.

UNITED STATES PATENTS 2,870,485 1/1959 Jones 75/O.5 B 2 Claims, 8Drawing Figures COPPER OXIDE HOLLOW PARTIALLY .OXIDIZED Cu POWDER- DARKLINES REPRESENT c11 0 AREA WITHIN DARK LINE IS Cu (50X) PAIENTEDncI 16I975 V 3.765866 CRUSHER W REDUCING CHAMBER FIG. IA FIG- IB FIG- lCNVENTOR 3, MW BY Mo m f 0 1104 ATTORNEYS PATENTEDUBHBIBIS 3.766666 SHEET2 BF 2 GU20 LAYER PARTIALLY DESULFURIZED Cu cRoss-sEcT "CRUDE" MIcTRUCTURE- EU TIC Cu +o+s DER NET K OF -cu 0- S FIG. 20 I 'F|G.2b,

Cu OXIDE (IOOXT 7 FIG. 2d PARTIALLY OXIDIZED Cu POWDER-- I DARK LINEsREPRESENT Cu O v AREA WITHIN DARK LINE IS Cu (50x) FIG. 26

4/ INVENTOR, y a g/l q m BY 6/ ATTORNEYS Cu TICLES AFTER CR NG.COVEREDWITH PRODUCTION OF COPPER AND COPPER OXIDE POWDER FOR POWDER METALLURGYRELATED APPLICATION The present application is a division of Ser. No.758,449, filed Sept. 9, 1968, and now U.S. Pat. No. 3,551,136.

BACKGROUND AND SUMMARY OF THE INVENTION The present invention relates topowder metallurgy and, more particularly, to the production of copperbase (including pure copper and copper alloy) powder of the type whichmay be blended, compacted, and sintered in the production of mechanicalparts andthe like. For example, bronze self lubricating bearings containby total weight approximately 90 percent copper powder and percent tinpowder. Also, high strength iron-base powder metallurgy parts contain bytotal weight from 1 to 25 percent copper powder and a remainder of ironpowder. The present invention also relates to the production of finecopper oxide powder.

Desirable characteristics of copper base powder for powder metallurgyapplication are as follows. Powder metallurgy parts require low costcopper powder. Such low cost must result from production utilizing lesspower, fewer production steps and less human attention than currently isused. The composition preferably is fairly pure although in someapplications, minor amounts of iron, manganese, carbon, etc. aredesirable but not necessary. The composition must be such as to havehighly compressible powder at room temperature.

Preferred dimensions range between 5 and 250 mi-' crons in diameter, theshape being irregular but equiaxial. Preferably the surface is rough inorder to provide low apparent density and high green strength.

A variety of previous processes have been employed in the production ofcopper powder. These processes include atomization of copper melt,hydrogen reduction of copper mill scale, electrolysis and leaching. Theatomization process, which is most typical, involves the followingsteps. A stream of first grade, scrap copper melt is atomized into smallsolid spherical copper particles by an impinging high pressure airstream projected from a nozzle attached to an air compressor. Thetypical air pressure required is about 250 pounds per square inch. Theresulting particle size depends upon melt temperature, melt viscosity,melt surface tension and air pressure. Usually the apparent density ofsuch solid spherical particles is greater than 4.5 grams per cubiccentimeter. The resulting particle size range is very wide so that theparticles must be screened into four or five mesh fractions for furthertreatment. The various mesh fractions, under heat, are oxidizedseparately in air or oxygen. Mesh fractions containing the largerparticles generally require longer oxidizing times and highertemperatures than mesh fractions containing the smaller particles. Sincethe fully oxidized copper base powder is very brittle, it can be crushedinto smaller irregular equiaxed copper oxide particles. Finally, aftersizing, the crushed oxidized particles are reduced with hydrogen toprovide pure and relatively fine copper particles suitable for powdermetallurgy parts. Since a large proportion of allcopper used ultimatelyis reclaimed as scrap, it is desired that existing atomization processesbe improved and made more economical. As is apparent, the elimination ofone or more of the foregoing steps in an atomization process and/or thereduction in cost of one or more of the foregoing steps will reduce theoverall copper powder production cost.

The primary object-of the present invention is to provide an improvedprocess, for producing'copper base or copper oxide powder, comprisingthe steps of first sulfurizing the copper base melt, next atomizing byany standard technique the sulfurized copper melt in oxygen to producehollow droplets by sulfur dioxide formation, next solidifying the hollowdroplets to form hollow shells and weakening the grain boundaries of theshells by sulfur dioxide and copper oxide formation at the grainboundaries, then crushing the weakened shells to form particles andfinally reducing the particles with hydrogen as desired. Under certainconditions, weak grain boundaries can be produced during solidificationto form hollow shells, thus eliminating the subsequent steps and thusdirectly crushing the solid hollow shells.

Other objects of the present invention will in part be obvious and willin part appear hereinafter.

The invention accordingly comprises the processes.

characterized by the steps and relationships which are set forth in theaccompanying disclosure, the scope of which will be indicated in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS For a fuller understanding of thenature and objects of the present invention, reference is made to thefolv lowing detailed description taken in connection with theaccompanying drawings, wherein:

FIG. 1 is a series of flow diagrams of processes illustrating thevarious steps of the present invention; and

FIG. 2 illustrates physical details of copper containing particlesundergoing a process of the present invention.

DETAILED DESCRIPTION FIG. 1 represents flow diagram illustrating thevarious steps and the various combinations of steps of the process ofthe present invention. It is clear from the FIG. 1 that variouscombinations of the steps provide slightly different processes toproduce the powder metallurgy grade copper powder and copper oxidepowder. Variations of the process of FIG. 1 are shown in FIGS.

' 1A, 1B, 1C and 1D, wherein analogous steps are designated by analogousnumerals and appropriate letters. The main and common step in FIGS. 1A,1B, 1C and 1D is the atomization of a sulfurized copper melt 10 byimpinging a molten stream from the melt with an air stream 13 underpressure, giving hollow particles of low apparent density and highspecific surface area due to the formation of SO within the particles.

FIG. 1A illustrates partially desulfurized hot hollow particles 11 beingfed directly into a vertical tube 12 with a constant temperature zone ofapproximately 1,000 1050C. As the hollow particles pass through thezone, an exothermic reaction between copper oxide and copper sulphide atgrain boundaries occurs by which apparent density is further decreaseddue to escape of S0 formed at grain boundaries and the grain boundariesof the particles are weakened by this further desulfurization. Theparticles then pass through a horizontal belt furnace 14 wherepreferential oxidation occurs at the grain boundaries. The hollow butpartially oxidized particles are fed into a crusher 16. The

crushed and partially oxidized particles are fed through a fluidizedchamber 18 or a normal horizontal belt furnace where they are reduced byhydrogen. The final product is a copper powder 19 suitable for powdermetallurgy parts. It has been found that by controlling melt sulfurcontent, melt temperature and atomization conditions, a variety ofdifferent hollow particles can be achieved with selected shapes, sizes,apparent densities and microstructures. In turn, the size, shape andapparent density of the final product can be controlled. Analternatively possible specific particle shape is characterized byrelatively flat minute platelet. In the foregoing process, sulfurintentionally is added to produce hollow particles of low apparentdensity, high specific surface area, and a unique composition. Theparticular element, sulfur, is selected because it can be removed easilyduring subsequent treatment. The process of adding sulfur is such as toproduce particles of desired shape, size, microstructure, composition,wall thickness, etc.

Generally, the melt to be atomized ranges in temperature between theliquidus temperature of the copper sulfur alloy and 2600F and contains0.05 to 3 percent sulfur with a remainder of copper, minor proportionsof other alloying materials optionally being present. The air streamimpinging on the molten copper stream ranges in pressure between and 500pounds per square inch. During atomization, the hollow particles formedby oxidation of sulfur to sulfur dioxide, acquire very rough interiorsurfaces. The high pressure sulfur dioxide gas formed in each moltendroplet inflates the droplet to a hollow shell, which in most casesbursts and the thus formed sulfur dioxide escapes. A particle in thiscrude form is shown schematically in FIG. 2A. The outer surface of theparticle has a thin copper oxide layer. The material beneath this copperoxide layer is essentially partially desulfurized copper. Themicrostructure of this partially desulfurized copper is shown in FIG. 23as being composed of a eutectic network of Cu, Cu S and Cu O distributedalong the grain boundaries.

The melt sulfur content, melt temperature, air pressure and otheratomization variables are controlled to give the following. The specificoxygen to sulfur ratio is in the range of 0.221 to 20.0:1 but preferablybetween 0.5:1 to 5:1. This particular ratio ensures removal of thesulfur by its combination with the oxygen that is available within theparticle during subsequent heating treatment. The apparent density ofthe atomized particles preferably is below 3 grams per cubic centimeterand for best results below 2 grams per cubic centimeter. By virtue ofits interior surface irregularity and hollowness, the surface area ofthe particle is large in relation to its mass, i.e. is large compared tothe surface of a normal solid particle of like material. Preferably allor most, say at least 70 percent of the Cu S and Cu O is at the grainboundaries in the eutectic network. Preferably the grain size of theparticle before crushing is approximately equal to the final size of theparticle desired. The wall thickness of the hollow particle is verysmall, preferably less than 250 microns thick. The hollow particle sizeand the wall thickness are quite uniform from particle to particle.

As indicated above and shown in FIG. 2B, the grain boundaries arecomposed of a eutectic network of Cu, Cu O and Cu S. The melting pointsof Cu, Cu-Cu O eutectic and Cu-Cu S eutectic respectively are 1,083,

1,065 and l,067C.' It is to be noted that the melting points of Cu-Cu Oeutectic and Cu-Cu S eutectic are lower than the melting point of pureCu. It is also to be noted that the following reaction is exothermic:

If particles with this type of microstructure are heated at anytemperature between some threshold temperature and 1,050C, the abovereaction will occur. Optionally this reaction can be conducted in avacuum. Since the above reaction is exothermic and if it is oncetriggered, the heat of reaction will raise the temperature at the grainboundaries to some temperature above that within the grain. If theparticles are heated to say I,O50C, the temperature in consequence israised within the grain boundaries to approximately 1,070C, therebycausing melting along grain boundaries. The sulfur dioxide erupts withinthe grain boundaries and, as it tries to escape, weakens the grainboundaries. At this point the escape of SO further decreases theapparent density of the hollow particles by as much as 25 percent.

FIG. 1B is similar to FIG. 1A except that furnace I4 is eliminated. FIG.1C is similar to FIG. 1A except that vertical tube furnace 12 iseliminated. FIG. 1D is similar to FIG. 1A except the furnace l4 and thevertical tube furnace 12 are eliminated.

In the step of oxidizing, the hollow particles are heated in closedfurnace 14, which is filled with air at 400-800C. In an alternativefurnace, the ends are open to air. If the hollow particles are left inthe furnace until the grain boundaries are completely oxidized, theresulting structure as shown in FIG. 20 is weak and brittle at the grainboundaries and the particles can be removed before the essentially purecopper composition of each grain is changed.

With respect to the crushing step, the hollow particles with weak andbrittle grain boundaries now can be easily crushed into much smallerparticles using a suitable crushing device. The resulting crushedparticles, as shown in FIG. 2d, are essentially equiaxed with rough andirregular surfaces. The crushed particles are much smaller than theatomized hollow particles. The interior of each crushed particleessentially is pure copper whereas the surface is covered with a thincopper oxide layer.

With respect to reduction of the crushed particles, the oxygen can beremoved by reduction with hydrogen in a fluidized bed or in aconventional reduction furnace. This latter reduction step can becompletely eliminated if annealing is properly controlled, i.e., ifannealing is completed in such a way that all oxygen and sulfur iseliminated at the grain boundaries during heating before crushing.

EXAMPLE I The following example will further illustrate the presentinvention. A quantity of copper having 0.5 percent sulfur by totalweight was melted in a crucible at 2,200F. The crucible had a A; inchhole at its bottom. A molten stream of metal flowing out of the hole wasimapac ted with compressed air at a pressure of pounds per square inchand a flow rate of 100 cubic feet per minute at room temperature andnoramal atmospheric pressure. The resulting particles, which werehollow, had an apparent density of approximately 2.5 grams per cubiccentimeter. The wall thickness of the particles typically was between0.004 inch and 0.010 inch. The outside diameter of these particlesvaried between 0.010 inch and 0.10 inch. The inside surface of theseparticles was very rough and composed of ridges and valleys" giving avery high specific surface area. The solid portion of the particles hada eutectic network of copper, copper oxide, and copper sulphide withoxygen to sulfur ratio of about 4 to 1. These particles were heated inair for one-quarter hour at 800C in a stainless steel rotatingcontainer. The particles were cooled, crushed and reduced at 450C forone-half hour under hydrogen to give fine copper powder suitable forpowder metallurgy parts.

Copper oxide particles also can be produced by the above process. Forthis purpose, the high specific surface particles produced by theatomization step described above are oxidized in a horizontal furnace.These hollow particles are oxidized in air at a temperature of between600 and l,050C for a time sufficient to oxidize the particlescompletely. This oxidation proceeds in two distinctly different'ways.First, the exterior and interior surfaces are oxidized and the resultingcopp r oxide-copper interfaces, one from outside the particle and theother from inside the particle, move toward each other. Secondly, Cu Sand Cu O at the grain boundaries combine to produce sulfur dioxide,which escapes to the atmosphere via the grain boundaries. These grainboundaries become more and more open so that oxygen can travel ordifiuse through thegrain boundaries, thereby greatly enhancing theoxidation rate. This enhanced oxidation rate reduces the total timeneeded for completely oxidizing a given weight of copper. Finally thehollow oxidized copper shells are crushed as desired in any suitablecrushing equipment.

EXAMPLE 2 The following example illustrates a production of fine copperpowder in accordance with present invention.

The process of Example 1 was repeated except the hol-.

low particles were instantly heated at 1,050C for about 1 minute. Thiscaused weakening of the grain boundaries due to formation of S and itsescape via the totally melted grain boundaries. The particles then wereheated in air at 800C for 5 minutes to oxidize the erupted grainboundaries completely. The particles were cooled, crushed and reducedunder hydrogen for one half hour at 450C to give fine powder metallurgygrade copper powder.

EXAMPLE 3 Same process as in Example 1, except that the sulfur contentwas 2 percent by total weight, the melt temperature was 2,400F and theair pressure was 200 pounds per square inch. The resulting particles hadan apparent density of 2 grams per cubic centimeter with thinner wallsand a thicker eutectic network as compared to that in Example 1. Theparticles were crushed and heated under hydrogen to remove excess oxygenand sulfur.

EXAMPLE 4 The following example illustrates a production of copper oxideparticles in accordance with the present invention. Example 1 wasrepeated except that the hollow particles here were heated in the air at1,050C for 5-7 minutes in order to achieve complete oxidation. Theparticles then were crushed to provide fine copper oxide powder.

In the foregoing examples, the melt was composed of essentially purecopper. it is to be understood that generally the present invention isapplicable to a copper base melt containing copper as its characteristicingredient (at least 50 percent by total weight), from 0.2 to 2 percentsulfur, and a remainder of an alloying metal, such as tin, nickel andzinc, having a heat of formation and a negative free energy of formationof its oxide and its sulfide, which are sufficiently low with respect tothose of SO that appreciable quantities of SO are formed sufficientlyquickly to effect the S0 puffing herein described.

The present invention thus provides an efficacious processfor producingcopper and copper oxide powder in an unprecedently inexpensive way.Since certain changes may be made in the disclosure hereof withoutdeparting from the scope of the present invention, it is intended thatall matter contained in the foregoing description or shown in theaccompanying drawing be interpreted in an illustrative and not in alimiting sense.

What is claimed is:

l. A mass of low apparent density copper base particles with highspecific surface area, equiaxed particle shape, high compactability andin the 5 to 300 micron size range, said copper base particles beingcharacterized by an open hollow shell having an exterior surface with athin copper oxide layer, an interior surface that is rough, and acomposition containing as its characteristic ingredients sulfur, oxygenand a remainder of copper, there being from 0.05 to 3 percent sulfur,the specific oxygen to sulfur ratio ranging between 0.2:1 and 20.0:1,said composition containing a eutectic network of Cu, Cu S and Cu O, theapparent density of the atomized particles ranging below 3 grams percubic centimeter, at least percent of said Cu S and 01 0 being at grainboundaries in said eutectic network.

2. The mass of low apparent density copper base particles of claim 1wherein the wall thickness of said hollow shell characteristically isless than 250 microns thick.

2. The mass of low apparent density copper base particles of claim 1wherein the wall thickness of said hollow shell characteristically isless than 250 microns thick.